The present invention relates to bridge rectifiers for use in telephones, more particularly to a transistorized bridge rectifier with overcurrent protection capable of being integrated monolithically and used to join the electronic circuits of a telephone subscriber set to a two-wire telephone line and having a low supply voltage.
In order for the electronic circuits of telephone sets to function properly, there must be applied to the terminals thereof a supply voltage having a prespecified and constant polarity and a value ranging between restricted and precise limits.
The polarity of the terminal voltage of a two-wire telephone transmission line is not prefixed, since during maintenance operations or repairs accidental polarity reversals may occur. Therefore, the electronic circuits of telephone sets must be joined to the two-wire telephone line by means of a circuit adapted to rectify the terminal line voltage when its polarity is reversed with respect to the required polarity.
In the event of an overcurrent on the line, regardless of how it is determined, the consequent voltage drop across the circuit can be detrimental to the electronic circuit connected thereto. To prevent that from happening, it is advisable to provide overcurrent protection.
The most widely used transistorized rectifier circuits are those using a "Graetz bridge" circuit arrangement which, with the addition of a limited number of components, can also protect the electronic circuits connected thereto from the effects of overcurrents on the line.
A bridge rectifier of known construction, as shown in FIG. 1, consists of a bridge structure comprising first and second bipolar p-n-p transistors, denoted by T1 and T2, and third and fourth n-p-n transistors, indicated by T3 and T4. The collector of T1 is connected to the collector of T2, and the collector of T3 is connected to the collector of T4, said connections respectively forming a first terminal denoted by the "+" sign, and a second terminal indicated by the "-" sign, to which connections the telephone circuit C which is to be energized is coupled.
The emitter of T1 and that of T3 are connected to the T-wire (tip wire) of a two-wire telephone line, the emitter of T2 and that of T4 being connected to the R-wire (ring wire) of the same telephone line.
The base of T1 and that of T3 are respectively connected to the R-wire of the line through a resistor R1 and a resistor R3; the base of T2 and that of T4 are respectively connected to the T-wire of the line through a resistor R2 and a resistor R4. These resistors serve to properly bias the bridge transistors which, under normal service conditions, operate at saturation. The second terminal "-" is also connected to the anodes of first and of second Zener diodes Z3 and Z4, whose cathodes are respectively connected to the T-wire and R-wire of the line. For a given polarity of the line, only the p-n-p transistor whose emitter is connected to the terminal of the line at a higher potential and the n-p-n transistor whose emitter is connected to the terminal at a lower potential are in a conducting state. The other two transistors are in a non-conducting state. Therefore, the supply current of the telephone circuit C, independently of the effective polarity of the line, always flows through the circuit from the terminal formed by the connection between the collectors of the two p-n-p transistors to the terminal formed by the connection between the collectors of the two n-p-n transistors, and the polarity of the voltage between the two terminals is constant.
A possible overcurrent in the line determines an increase in the total voltage drop across the rectifier circuit.
However, as soon as the voltage across the poles of the Zener diode whose cathode is connected to the wire of the line with a higher potential equals the breakdown voltage V.sub.Z of the junction of the diode, the Zener diode is switched to the reverse conducting state by the Zener effect. The other Zener diode, instead, starts to conduct as a normal diode as soon as the voltage across its poles is equal to the threshold voltage V.sub.ONZ with respect to forward conduction. Therefore, the total voltage drop across the rectifier circuit does not exceed the maximum value: EQU V.sub.R MAX =V.sub.Z +V.sub.ONZ
when overcurrents occur on the line.
The maximum voltage applied to the electronic telephone circuit does not exceed the maximum value: EQU V.sub.C MAX =V.sub.Z -V.sub.CE sat
wherein V.sub.CE sat is the collector-emitter saturation voltage of a p-n-p bridge transistor.
Both the rectifier circuit and the telephone circuit are thus protected against overcurrents.
However, a rectifier circuit of the type described hereinabove, with overcurrent protection, is not the best solution from the economic point of view.
Indeed, the introduction of supplementary circuit elements, such as the two Zener diodes, in addition to those of the bridge, means an increase in the "cost" of the circuit (whether realized with discrete components or integrated monolithically) for reasons of space, well known to workers in the art, reserved for integration and processing technology.